XL: Rename, cleanup and add more comments. (#1769)

- xl-v1-bucket.go - removes a whole bunch of code.
- {xl-v1,fs-v1}-metadata.go - add a lot of comments and rename functions
   appropriately.

docs/backend/xl/uploads.json

@@ -0,0 +1,10 @@
+{
+    "version": "1",
+    "format": "xl",
+    "uploadIds": [
+        {
+            "uploadID": "id",
+            "startTime": "time",
+        }
+    ]
+}